Phosphites and other organic phosphorus compounds are used as antioxidants and stabilizers in organic materials such as organic polymers. In organic polymers such as polyolefin homopolymers and copolymers, they are generally considered better than phenolic antioxidants at elevated temperatures due to their ability to destroy hydroperoxides which decompose and lead to autooxidation chain reactions. Thus, organic phosphorus compounds are important for oxidative stability during numerous operations such as polyolefin extrusion. Additionally, in organic polymers such as polyvinyl chloride, organic phosphites are used to improve resistance of the resin to discoloration on exposure to the action of heat or light.
Thus, additives of the organic phosphite type are needed in larger amounts for processing organic polymers. Many of the antioxidants and stabilizers employed in organic materials have limited effectiveness or impart undesirable properties such as discoloration. Consequently, there exists a need for novel organic phosphites with improved antioxidant and stabilizing properties. The aryl fluorophosphites of the present invention allow organic materials to maintain excellent color and thermal stability.
Aryl fluorophosphites and their use as antioxidants in polymeric compositions have been disclosed in the patent literature. For instance, U.S. Pat. No. 4,912,155 discloses a large number of substituted phenyl and diphenyl compounds, including several dialkylphenyl fluorophosphites. However, the disclosure of this patent indicates that the synthesis of such compounds required a difficult, time-consuming process which also suffered from low yields. For instance, the synthesis of bis(2,6-di-t-butylphenyl) fluorophosphite required preparation and isolation of the corresponding chlorophosphite, and then fluorination of the chlorophosphite in a process which required a total of 61 hours reaction time and even then provided the target fluorophosphite in a yield of only 23%.
This patent also discusses preparation of bis-(2,4-di-t-butylphenyl) fluorophosphite by reaction of phosphorus trichloride and 2,4-di-t-butylphenol in the presence of a stoichiometric amount of triethylamine, followed without isolation of the intermediate by reaction with antimony trifluoride to convert the chloro groups to fluoro. This disclosure also involved a time-consuming process to the desired aryl fluorophosphite with a yield of only 55%.
Thus, there remains a need in this field for improvements in the processes of synthesizing aryl fluorophosphites. The identification of a catalyst or catalyst system would seem to be one way to meet this need, but catalysts which are satisfactory in all respects have not been found prior to the present invention. European Patent Application publication number 312,915 discloses triethylamine as a catalyst for synthesis of chlorophosphites, but as noted above, triethylamine used even in stoichiometric quantities did not lead to satisfactory reaction times and yields in the production of aryl fluorophosphites. U.S. Pat. No. 5,061,818 discloses the use of hydrogen halide salts of pyridine as a catalyst for similar syntheses, but pyridine is known to impart an extremely unpleasant odor to products containing it. Thus, a process using pyridine would require additional precautions to be taken in materials handling and process equipment. Also, additional purification steps would be required after formation of the fluorophosphite to avoid the presence of even the minutest amounts of pyridine in products to which the fluorophosphite is to be added.